1. Field of the Invention
The invention concerns an electroluminescent device and more particularly an electroluminescent device fabricated from porous silicon material.
2. Discussion of Prior Art
Light emitting devices may emit light by a variety of processes. A conventional tungsten wire light bulb emits visible light when an element in the light bulb reaches a certain temperature. The emission of visible light from a substance at high temperature is termed incandescence. Luminescence is a phenomenon distinct from incandescence and is produced when electrons lose energy radiatively when moving from an excited energy state to a lower energy state which may be their ground state. Photoluminescence is luminescence from electrons which are excited into a high energy level by the absorption of photons. Photoluminescent porous silicon is described in U.S. Pat. No. 5,438,618. Electroluminescence is luminescence from electrons which are excited to higher energy levels by an electric field or an electric current. An example of electroluminescent porous silicon is described in United Kingdom Patent No. GB 2268333 B.
Light emitting diodes are an important class of electroluminescent devices. Light emitting diodes are commonly fabricated from semiconducting materials in the Ga1-xAlxAsGa1-xInxAs1-yPy, and GaAs1-xPx systems. A measure of the efficiency of a light emitting diode is its external quantum efficiency, which is defined as the number of photons emitted by the diode divided by the number of electrons entering the diode. Devices fabricated from these materials may have external quantum efficiencies greater than 10%. Electroluminescent devices fabricated from GaAs compounds suffer from the disadvantage that they are difficult to integrate monolithically with silicon based integrated circuit technologies. It has for many years been an important objective of workers skilled in the field of semiconductor technology to be able to produce an electroluminescent device which is compatible with silicon based integrated circuit technologies.
A world-wide interest in the possible use of porous silicon as a luminescent material in an electroluminescent device was generated by a paper by L. T. Canham in Applied Physics Letters, Volume 57, Number 10, 1990, pp 1046-1048. This paper reported efficient visible photoluminescence from quantum wires in porous silicon at room temperature. A silicon quantum wire may be defined as a physically continuous column of silicon of width not greater than 10 nm, having a length which is not less than twice its width, and whose boundaries are surrounded by a suitable passivation layer. A porous silicon electroluminescent device offers the advantage of potential compatibility with conventional silicon integrated circuit fabrication techniques for use in applications such as optical displays and optoelectronic integrated circuits.
As mentioned above, electroluminescent porous silicon is described in United Kingdom Patent No. GB 2268333 B. The world-wide interest in electroluminescent porous silicon has been evidenced by a large number of published scientific papers which describe electroluminescent devices incorporating porous silicon. However, the luminescence efficiencies reported for these devices have been disappointingly low. V. P. Kesan et al. in Journal of Vacuum Science and Technology A, Volume 11, Number 4, 1993, pp 1736-1738 have reported a p-n junction porous silicon electroluminescent device having an efficiency in the range 0.04% to 0.1%. The Kesan et al. device however has a threshold current density of 30,000 Amxe2x88x922 before electroluminescence is observable. Such a high threshold value would seem to be incompatible with the stated efficiency values. Also, there is no indication in the paper of Kesan et al. as to whether the quoted efficiency measurement is an external quantum efficiency figure or some other efficiency, such as internal quantum efficiency. If the quoted efficiency figure is an internal quantum efficiency, the external quantum efficiency figure would be significantly lower, perhaps of the order of a factor 10 lower.
F. Kozlowski et al. in Sensors and Actuators A, Volume A43, No. 1-3, 1994, pp 153-156 report a light emitting device in porous silicon having a quantum efficiency of 0.01%. This paper however only provides details of the electrical characteristics of luminescent devices having quantum efficiencies in the range 10xe2x88x923 to 10xe2x88x924%.
L. V. Belyakov et al. in Semiconductors, Volume 27, No. 11-12, 1993, pp 999-1001 have reported luminescence efficiencies of up to 0.3% for cathodically biased electroluminescent porous silicon devices incorporating a liquid electrolyte. They reported the observation of electroluminescence at a current density of 200 Amxe2x88x922. A device incorporating a liquid electrolyte would be difficult to integrate with a conventional silicon based microcircuit.
W. Lang et al. in Journal of Luminescence, Volume 57, 1993, pp 169-173 describe an electroluminescent device which has a thin gold top electrode. Lang et al. observed electroluminescence above a current threshold of 1.1 Amxe2x88x922 and measured an external quantum efficiency of 0.01%. They estimate that their device had an internal efficiency which was greater than 0.1%. An external efficiency value is a measure of the efficiency of generating photons external to a device and is distinct from internal efficiency values which are measures of the efficiency of generating photons within the device. The internal efficiency value will be higher than the external efficiency value because of internal absorption and scattering mechanisms.
Virtually all scientific papers published on porous silicon light emitting diodes have been concerned with device performance during operation in ambient air. An exception to this is a paper by Badoz et al. published in Proceedings 7th International Symposium on Silicon Materials Science and Technology, Electrochemical Society Inc. Pennington, N.J., Proc. Volume 94-10, pages 569-574D, 1994. They demonstrate that the stability of inefficient (external quantum efficiency 10xe2x88x924%) porous silicon light emitting diodes is dramatically improved when operated in dry nitrogen gas rather than ambient air. They suggest that degradation arises from electrically enhanced oxidation of the silicon skeleton.
Scientific papers have been published which suggest that when p-type silicon is anodized n-type porous silicon is produced. N. J. Pulsford et al. in Journal of Luminescence, Volume 57, 1993, pp 181-184 reported the anodization of 25 xcexa9cm p-type silicon substrates to produce photoluminescent porous silicon. From measurements of the electrical characteristics of the porous silicon, they came to the conclusion that their results were consistent with the porous silicon being n-type. Amisola et al. in Applied Physics Letters, Volume 61, Number 21, 1992, pp 2595-2597 reported scanning tunnelling microscopy measurements of porous silicon produced from p-type silicon which showed that at least the surface of the porous silicon behaved like n-type material. Measurements of the spreading resistance of a layer of porous silicon having a porosity of 30% produced from heavily doped p-type silicon, using a method described in U.S. Pat. No. 5,348,618, show that the spreading resistance of the porous silicon increases with increasing depth. This corresponds to an increase in resistivity with increasing depth. This is opposite to the behaviour of porous silicon produced from heavily doped n-type silicon, and is indicative of a n-p junction being formed at the porous siliconxe2x80x94silicon interface. It is concluded that previously published work describing the production of electroluminescent devices by the anodization of p-n silicon structures does not result in a p-n junction being formed within the porous silicon at a position corresponding to the original p-n interface but instead results in a heterojunction between the porous silicon and the bulk silicon.
It is an object of the invention to provide an alternative electroluminescent device.
The present invention provides an electroluminescent device biasable to produce electroluminescence and comprising an electroluminescent porous silicon region and electrical connections to the porous silicon region, characterized in that electroluminescence from the porous silicon region is detectable when the device is biased such that a current having a current density of less than 1.0 Amxe2x88x922 flows through the device.
The invention provides the advantage that a low threshold current is required to produce electroluminescence. A low threshold current is advantageous in applications where power conservation is at a premium, for example battery powered electronics.
The devices of the present invention may be fabricated by a method which includes anodizing a silicon wafer after it has received a dopant implant but without the wafer being annealed after the implantation. In general, silicon wafers are annealed after they have received a dopant implantation in order to activate electrically the dopant species and to anneal any damage to the crystal structure caused by the implantation process. Anodizing a wafer after a dopant implantation with no intervening anneal stage would be considered surprising to those familiar with silicon processing techniques.
The device may exhibit electroluminescence when biased such that an electrical current having a current density of less than 0.1 Amxe2x88x922 flows through the device. Electroluminescence from the device may be visible to an unaided human eye when the current density is less than 0.1 Amxe2x88x922. Electroluminescence may be detectable when the current density is less than 0.01 Amxe2x88x922 and as low as 0.0001 Amxe2x88x922. The external quantum efficiency of the electroluminescence may be greater than 0.1%. External quantum efficiencies as high as 0.4% have been measured for devices operating at 200 K (xe2x88x9273xc2x0 C.). The combination of high efficiency and low threshold current are particularly advantageous. A device of area 1 mm2 operating under an applied bias current density of 0.0001 Amxe2x88x922 would require a bias current of only 10xe2x88x9210 amps, or 0.1 nA, to produce detectable luminescence.
In another aspect, the invention provides a solid state electroluminescent device comprising an electroluminescent porous silicon region and electrical connections to the porous silicon region, characterized in that the device is biasable to produce electroluminescence from the porous silicon region with an external quantum efficiency greater than 0.01%.
A high external quantum efficiency is advantageous since for a given luminescent intensity, the more efficient a device, the less power it requires.
The solid state device of the invention may exhibit electroluminescence with an external quantum efficiency greater than 0.1%. The external quantum efficiency may be in the range 0.01% to 0.18% and may be at least 0.4%.
The solid state device of the invention may comprise a p-type porous silicon region and an n-type porous silicon region with a p-n junction therebetween. As stated previously, there are indications that conventionally produced porous silicon is n-type even if the starting material was p-type silicon. It therefore follows that previous electroluminescent porous silicon devices which allegedly contained a p-n junction within porous silicon may have had some other form of junction either between a top contact and the porous silicon or at the interface between the porous silicon and the unanodized bulk silicon.
At least one of the p-type and n-type porous silicon regions may be surface doped. Surface doped porous silicon is porous silicon which has been doped by dopant species deposited on the surfaces of the silicon structures forming the porous silicon. These dopant species may either remain at the surface or diffuse into the silicon. The p-type porous silicon may be surface doped, and the surface dopant may be boron. The device may electroluminesce with an external quantum efficiency greater than 0.1%.
The device of the invention may incorporate an injector layer for injecting holes into a luminescent region of the porous silicon. This injector layer may be a surface layer of porous silicon. It has been found that the surface region of the porous silicon may have raised levels of oxygen, carbon, and fluorine and so may have a wider band gap than the luminescent region of porous silicon and so would act as an efficient hole injector.
Preferably the device of the invention is produced by light assisted anodization in aqueous hydrofluoric acid. It is known that light assisted anodization in ethanoic hydrofluoric acid generally results in mesoporous porous silicon. Mesoporous porous silicon has pore sizes greater than 20 xc3x85 wide and less than 500 xc3x85 wide. It is known that light assisted anodization of an nxe2x88x92 silicon substrate may also generate some degree of macroporosity. Macroporous porous silicon has pore sizes greater than 500 xc3x85 wide. The anodization conditions of the device of the invention avoid the creation of both macroporous and mesoporous porous silicon. The active portion of the device is microporous with pore sizes less than 20 xc3x85 wide.
The electroluminescent device may comprise an n-type bulk silicon region, an n-type porous silicon region adjacent the n-type bulk silicon region, a p-type porous silicon region adjacent the n-type porous silicon region, and electrical contacts to the bulk silicon region and the p-type porous silicon region.
The device of the invention may be operable to produce a modulated light output. The light output may be modulatable at a frequency greater than 10 kHz. Modulation of the optical output of the device has been observed at modulation frequencies up to 1 MHz. The device of the invention may have an electroluminescence intensity maximum at a wavelength which is greater than 400 nm and less than 900 nm. The intensity maximum may be at a wavelength in the range 520 nm to 750 nm.
The device of the invention may be an encapsulated device, whereby the porous silicon is protected from the environment since the operating efficiency of an unencapsulated device may degrade upon exposure to water vapour and/or oxygen. The encapsulation may be provided by a vacuum chamber or some other form of encapsulation arrangement such as an impermeable top contact to the porous silicon which may be of indium tin oxide.
The device may be integrated with other silicon devices as part of an opto-electronic integrated circuit. The electroluminescent device of the invention may be combined with further devices of the invention to form a display which may produce a light output having a plurality of colours.
In another aspect, the invention provides a method of fabricating an electroluminescent device including the steps of:
(i) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;
(ii) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and
(iii) depositing an electrode on the porous silicon region;
characterized in that the surface region has a sheet resistivity greater than 100 xcexa9xe2x88x921 immediately prior to the anodizing step.
In a further aspect, the invention provides a method of fabricating an electroluminescent device including the steps of:
(i) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;
(ii) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and
(iii) depositing an electrode on the porous silicon region;
characterized in that less than 1% of the acceptor impurity is electrically active prior to the anodizing step.
In a further aspect, the invention provides a method of fabricating an electroluminescent device including the steps of:
(i) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity and at least a part of the region has an acceptor impurity volume concentration comparable with the solid solubility limit of the acceptor impurity in silicon;
(ii) anodizing the wafer under illumination to produce a porous silicon region extending through the surface region; and
(iii) depositing an electrode on the porous silicon region.
In a further aspect, the invention provides a method of fabricating an electroluminescent device including the steps of:
(i) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;
(ii) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and
(iii) depositing an electrode on the porous silicon region;
characterized in that the silicon wafer does not receive an anneal between steps (i) and (ii).
In a further aspect, the invention provides a method of fabricating an electroluminescent device including the steps of:
(i) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;
(ii) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and
(iii) depositing an electrode on the porous silicon region;
characterized in that the anodization step causes surface doping of silicon quantum wires within the porous silicon region, rendering the surface doped quantum wires p-type.
In a yet further aspect, the invention provides p-type porous silicon, characterized in that the porous silicon has a porosity greater than 30%. The porosity may be greater than 60% and the porous silicon may comprise quantum wires.
The invention further provides substantially wholly microporous visibly luminescent porous silicon, characterized in that the porous silicon is derived from n-type bulk silicon.
In another further aspect, the invention provides an electroluminescent device comprising a porous silicon region and electrical connections to the porous silicon region, characterized in that the porous silicon region contains a p-n junction therein.
The invention further provides an electroluminescent device comprising a porous silicon region and electrical connections to the porous silicon region, characterized in that the porous silicon region is a wholly microporous visibly luminescent region fabricated from n-type bulk silicon.